Preparation of aromatic hydrocarbons



Sept. 22, 1953 w. H. DAvls 2,653,175

PREPARATION oF ARoMATIc HYDRocARBoNs `Filed may 4, 1951 2 sheets-sheet 1 Sept. 22, 1953 w. H. DAvls PREPARATION OF AROMATIC HYDROCARBONS 2 Sheets-Sheet 2 Filed May 4, 1951 Patented Sept. 22, 1953 PREPARATION F AROMATIC HYDROCARBONS 'William H. Davis, Bala Cynwyd, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Application May 4, 1951, Serial No. 224,466

14 Claims. (Cl. 260-668) This invention relates to the preparation of -aromatic hydrocarbons of high purity. More particularly the invention is concerned with a process involving a series of interrelated steps vwherein petroleum stock s containing naphthenes are treated under catalytic conditions to convert the naphthenes to aromatics and the resulting' aromatics are recovered in purified form.

Aromatic hydrocarbons of high purity have numerous uses in the chemical industry. There is an increasing demand particularly for benzene, toluene and the xylenes as starting materials for synthesizing various compounds of commercial importance. It is known that aromatic hydrocarbons can be formed from the naphthene hydrocarbons occurring in petroleum by treatment of petroleum fractions with a dehydrogenating catalyst under conditions effective to withdraw hydrogen atoms from naphthene rings and thereby convert them to aryl rings. This type of reaction occurs in catalytic conversion operations commonly referred to as reforming. The resulting aromatic hydrocarbons, however, are associated with numerous non-aromatic components from which they must be separated if relatively pure aromatic products are to be obtained.

The present invention provides an improved process for forming aromatic hydrocarbons from petroleum stocks and separating them from associated non-aromatic hydrocarbons to obtain aromatic products of high purity. In one aspect the process of the invention involves distilling crude petroleum to produce two naphtha fractions for use as charge materials for separate dehydrogenating or reforming steps. One of the fractions has a boiling range such as to include the Cu and C1 naphthenes of the crude stock.

-The other is a heavier fraction which includes Cs naphthenes and boils essentially below the boiling range of xylenes. The lower boiling fraction is sent to a reforming operation wherein the contained naphthenes are catalytically converted to benzene and toluene. The heavier fraction is separately reformed under catalytic conditions to convert the naphthenes to xylenes. The reaction mixture from the second reforming step is distilled to remove the contained non-aromatic hydrocarbons which boil generally lower than xylenes produced and to yield a Xylene-rich product. Benzene and toluene of high purity are recovered from reaction product of-therst reforming step by treating such reaction product with an aromatic-selective adsorbent and subsequently displacing .the adsorbed benzene .and toluene by means of the Xylene-rich product obtained from the higher boiling charge fraction. Xylene retained by the adsorbent is thereafter displaced and recovered from it while the adsorbent is being re-used for further treatment of reaction product containing benzene and toluene.

In another modification of the process the lower boiling naphtha fraction which is reformed may contain C6 naphthenes Without substantial amounts of C1 naphthenes, in which case the main aromatic product of the process will be benzene and xylenes. In still another modification the lower boiling naphtha fraction may contain C7 naphthenes without substantial amounts of Cs naphthenes, the main aromatic products then being toluene and xylenes.

It will be understood that the term "xylenes as used herein refers to the Ca aromatic hydrocarbons in a generic sense and may include ethylbenzene in addition to ortho, meta and para xylenes.

The reforming steps of the process can be conducted with any suitable dehydrogenating catalyst which is effective to convert the naphthenes contained in the naphtha charge fractions to corresponding aromatic hydrocarbons. Numerous reforming catalysts are known for effecting this type of reaction. Examples are platinum or palladium deposited in minor proportion on a supporting component such as silica, alumina, magnesia or mixtures of such components; and the oxides of'molybdenum, vanadium or chromium, alone or in combination, together with a. supporting component. Preparation of the catalysts may include various known special treatments for improving their effectiveness for conversion of naphthenes to aromatics while minimizing undesired reactions such as cracking. It is also desirable that the catalyst selected be one which -is effective not only for dehydrogenating but also for isomerizing naphthenes which have rings oi five carbon atoms into six carbon ring compounds so that maximum yield of aromatics will be obtained. Such isomerizing ability is particularly desirable when benzene is to be one of the products, since most crude petroleums contain substantial amounts of methylcyclopentane which can, in addition to the contained cyclohexane, be converted into benzene if a suitable isomerizing and dehydrogenating catalyst is used.

Reforming of the naphtha fractions with catalysts of the foregoing types preferably should be done in the presence of free hydrogen in order to minimize cracking and coke formation. Hydrogen can be maintained in desired amount within the reaction zones by the usual reforming procedure of recycling to the reactors hydrogen separated from the bulk of the ,reaction product.

In the accompanying drawings Figure 1 is a diagrammatic owsheet illustrating the process of the invention and Figure 2 is a owsheet illustrative of a more specific manner of practicing the process.

Referring to Figure 1 the two naphtha fractions forv charging to the reforming or dehydrogenating steps may be prepared by fractionation of a crude oil in a distillation column I to which it is introduced via line Il. The lowest boiling, portions of the crude are distilled overhead through line I2 and heavy portions are removed from the bottom by means of line I3. A light naphtha fraction and a heavier naphtha fraction are obtained as sidestream products through lines I4 and I5 respectively. The process as illustrated in Figure 1 is conducted to produce both benzene and toluene in addition to xylenes; consequently the light naphtha fraction which flows through line I4 has a boiling range to include both the Cs and C1 naphthenes present in the crude charge. The heavier naphtha fraction obtained through line I5 has a boiling range such that the fraction includes Cs naphthenes present in the crude but its boiling range is not so high as to overlap substantially into the xylene boiling range. As examples of suitable boiling ranges for these fractions, it may be considered that the lower boiling naphtha includes the hydrocarbons boiling essentially from 150 F. to 225 F. and that the higher boiling naphtha includes those boiling essentially from 225 F. to 275 F.

Alternatively, if the process is to be operated for the production of benzene and xylene but not toluene, the lower boiling fraction obtained in line I4 may have a boiling range such as to include the Cs naphthenes and an intermediate fraction containing the C7 naphthenes may be withdrawn from column l0 through line I8 and valve I 1. For example. the lower boiling fraction may have a range of 150 F. to 185 F. and the higher boiling fraction a range of 225 F. or 235 F. to about 275 F. On the other hand, if the process is to be practiced for producing toluene and xylene but not benzene, then the distillation in column I0 may be conducted toremove all hydrocarbons boiling up through the Ca naphthene range through overhead line I2 so that the naphtha fraction withdrawn through line I4 will contain Cv naphthenes without substantial amounts of Ce naphthenes. In such case the lower boiling naphtha may have a range of say 185 F. to 230 F.

In the presentdescription the naphtha fraction obtained from column II through line I4 will be considered to include both Ca and C7 naphthenes. This fraction is sent to catalytic reformer I8 wherein it is reacted under conditions effective to produce benzene and toluene by dehydrogenation of the naphthenes. Preferably the catalyst is one which is effective also to isomerize naphthenes having rings of five carbon atoms, such as methylcyclopentane and dimethylcyclopentanes, into compounds of six carbon rings so that the maximum yields of benzene and toluene will be obtained.

The reaction mixture from catalytic reformer I8, which includes benzene and toluene in admixture with numerous non-aromatic hydrocarbons boiling generally below and within the f line I3 to a distillation operation as hereinafter described.

The heavier naphtha fraction, which is withdrawn from column I0 through line I5, is reacted in another catalytic dehydrogenatlng or reforming step illustrated by catalytic reformer 20. This operation may be conducted in the same reaction zone or zones as the reforming operation above-described in a blocked-out" type of operation or separate reactors may be provided so that each reforming step may be carried out concurrently and continuously. The conditions maintained in catalytic reformer 20 are such that the Ca naphthenes are dehydrogenated to produce mainly xylenes. Such xylenes generally will include a relatively small amount of ethylbenzene, and also small amounts of higher boiling aromatic products usually will Y be produced. Essentially all of the aromatics formed in this reaction will boil above the boiling range of the heavier naphtha charge and therefore can readily be separated from the associated non-aromatic components by distillation.

The reaction product from catalytic reformer 20 is passed through line 2l to distillation zone 22 wherein components boiling below the xylene boiling range are distilled and removed through overhead line 23. These components comprise the saturate hydrocarbons which were present in the charge fraction and also include any toluene that may have been present in the charge naphtha. From the bottom of tower 22 material boiling within the xylene boiling range and higher is removed through line 24 and panes to distillation column 25 for separation of the xylenes from any higher boiling aromatic components. A xylene-rich product is withdrawn overhead through line .28 and ows to xylene storage tank 21. The higher boiling material, comprising mainly C9 aromatics, are removed from the base of the column by means of line 28.

Referring back to the reaction product from the first reforming step, this material is introduced into distillation column 30 wherein those hydrocarbons which boil below the boiling point of benzene are distilled and removed through overhead line 3|. In one manner of practicing the process, the remainder of the reaction product may be withdrawn from the bottom of the column through line 32 and passed through line 33, valve 34 and line 35 into surge tank 38, valves 31 and 38 being closed. When the operation is conducted in this manner, the mixture fed to tank 33 will contain substantially all of the benzene and toluene produced during the reaction in reformer I8. In a preferred manner of practicing the process, however, the distillation in column 30 is conducted to produce a sidestream fraction which contains essentially all of the benzene but onlya part of the toluene and a toluene fraction of high purity is obtained from the bottom of column 30 through line 3l. This can be done by operating the column to cut between the sidestream and bottom products at a temperature of about 230 F. The sidestream fraction is sent through valve 38 and line 35 to tank 38, valve 34 being closed; while the bottom fraction is passed through valve 31 and line 39 as a toluene product of high purity. Operation in this manner is particularly advantageous in that only a portion of the total toluene produced need be passed through the .subsequently described puriiication steps.

The mixture in surge tank 38 containsv benzene range of the charge fraction, is passed 15113011811 .75 and toluene together with associated saturated hydrocarbons derived from the naphtha charged to reformer i8. In order to separate these saturate components from the aromatics the mixture is treated by means of a suitable aromaticselective adsorbent, such as silica gel or activated carbon, to selectively adsorb the benzene and toluene, and these aromatics are subsequently displaced from the adsorbent by means of xylene-rich material obtained from tank21. This may be done by first introducing some of the mixture from tank 36 through line 49, valve 4| and line 42 into a contact zone 43 which contains the aromatic-selective adsorbent. After a suitable amount of the mixture from tank 35 has been introduced into .the adsorbent, valve 4I is closed and xylene-rich material from tank 21 is passed into contact zone 43 through line 44, valve 45 and line 42 in order to displace the benzene and toluene. This adsorption-step may be conducted by employing a fixed bed of adsorbent or by continuous countercurrent adsorption. In the present ldescription the adsorption step will be considered as involving a cyclic operation utilizing a stationary adsorbent bed or a plurality of such beds in parallel flow arrangement to permit continuous passage of the charge and desorbent streams to the adsorption operaxylene-rich material from tank 21 is introduced.`

The saturate components of the charge mixture tend to pass through the adsorbent ahead of the benzene and toluene which are selectively retained and will therefore be removed from these aromatic constituents. Upon addition of the xylene material following the charge mixture displacement of benzene and toluene from the adsorbent occurs. The xylenes in turn are displaced by the benzene and toluene present in' the next portion of material introduced to contact zone 43 from tank 36.

During each cycle the eillux stream which passes from" contact zone 43 through line 46 is cut into two portions. One portion contains the benzene and toluene displaced from the adsorbent and also some xylene, and this portion is sent through valve 41 and line 48 to distillation column 49. Benzene and toluene are distilled overhead via line 50 while the xylene is obtained as a bottom product which is returned by means of lines and 52 to storage tank 21. The mixture of benzene and toluene from column 49 passes to column 53 for separating these components from each other. Purified benzene is withdrawn through line 54 as one product of the process. `Puried toluene is removed by means of line 55 as another product and may be mixed with the toluene `obtained from line 39.

In another, part of the cycle efflux from contact zone 43 will be composed essentially of xylene and saturate hydrocarbons derived from the mixture introduced from tank 36. At this time valve 41 is kept closed and the efliux is sent through valve 56 and line 51 to distillation column 58. saturate hydrocarbons are removed overhead through line 59, while the recovered xylene is withdrawn from the bottom of column 58- and passed through lines B9 and 52 back tol storage tank 21.

In order that the distillations carried out in columns 49, 53 and 5B will be continuous surge 6 of the materials to be distilled. This will permit a continuous supply of feed materials of substantially constant composition.

Figure 2 illustrates a more specific embodiment of the invention which is generally similar to that described in connection with Figure 1 but which includes several features that are highly advantageous for commercial practice. For purpose of describing the process of Figure 2 the lower boiling naphtha fraction which is reacted in the first reforming step will be considered as having a true boiling point range of'about 150- 225 F., while the higher boiling naphtha fraction reacted in the second reforming step will be considered as having a true boiling point range of about 240-275 F. The aromatic-selective adsorbent used for purifying the aromatic reaction products will be referred to as silica gel.

The naphtha fraction having a boiling range of 15G-225 F., which contains naphthenes having six and seven carbon atoms per molecule, is sent through line 1li to catalytic reformer 1i which contains a dehydrogenating and isomer'- izing catalyst such as, for example, platinum deposited in minoramount on an acid treated silica-alumina carrier component. Hydrogen produced by the dehydrogenation reaction and separated from the reaction products as hereinafter described is recycled through line 69 for admixture with the charge naphtha sent into the reactor. While in Figure 2 only one reaction zone is illustrated for conducting this reforming step, it will be understood that a plurality of reactors may be provided in series with suitable heating means between reactors to supply heat' for the endothermic dehydrogenation reaction and thereby maintain the desired temperature level. Suitable reaction conditions for promoting the desired dehydrogenating and isomerizing reactions generally will include a temperature within the range of 800-1000 F., a pressure of 150-400 lbs/square inch, a space rate of 1-4 liquid volumes of naphtha charge per volume of catalyst per hour and a hydrogen recycle ratio of 2-6 moles per mole of naphtha charge. More preferable reaction conditions include a temperature of 90o-950 F., a pressure of 250- 300 lbs/square inch. a space rate of 1.5-3 liquid volumes of naphtha charge per volume of catalyst per hour and a hydrogen recycle ratio of 3-4 moles per mole of naphtha charge.

Reaction mixture from reformer 1I is passed through line 12 and cooler 13 ino separator 14 wherein gaseous components are removed from liquid product. The gaseous product Hows through line 15 and a portion of it may be returned through valve 16 and line 69 to the reactor in order to maintain the desired concentration of hydrogen within the reaction zone. As is well known the presence of hydrogen in the reforming zone is 'highly advantageous in suppressing cracking reactions and minimizing coke formation. The remainder of the gaseous stream from separator 14 is passed through line 11 into absorber 18 wherein it flows 'countercurrent to xylene lwhich has been obtained from xylene tank 19 and supplied to the absorber by means of lines and 8|. 'I'he purpose of having absorber 18 in the system is to provide for removal of hydrogen and low boiling hydrocarbon gases, which are withdrawn through line 82, without inordinate loss of higher boiling hydrocarbons, particularly benzene and toluene. If desired, hydrogen may be recycled from the top of absorber 13 by opening valve 83 and closing valve 16, but

it is preferred to supply the desired hydrogen for the reforming operation in the manner previously described.

'I'he xylene which flows from the bottom of absorber 18 through line 84 contains hydrocarbons recovered from the gas stream and is introduced into distillation column 85 along with the liquid reaction product which iiows from separator 14 through line 86. The distillation in column 85 is conducted so as to remove all components boiling below 150 F. `through overhead line 81 and to obtain a sidestream fraction through line 88 which is essentially free of xylene. 'I'he xylene used as absorption medium is thus recovered through line 89 as a bottom product from tower 85. In practice of the process this material may also contain ,some toluene which is recovered therefrom by distillation as subsequently described.

The sidestream fraction flowing through line 88, which is composed essentially of benzene, toluene and saturate hydrocarbons, is passed through valve 90 and line'8l into adsorption zone 82 for recovery of the benzene and toluene by selective adsorption on silicia gel. In the presently described practice of the process a low boiling saturate hydrocarbon such as butane is introduced into the silicia gel immediately following the sidestream fraction from line 88. The purpose of so introducing butane is to achieve a more effective separation of the benzene and toluene from associated saturate hydrocarbons by displacing such saturates from the adsorbed aromatics and toluene. The amount of butane employed in each cycle of operation should be suillcient to effect such displacement of the saturates without substantial desorption of benzene and toluene from the silica gel. The periodical addition of butane is done by withdrawing butane during a portion of each cycle from storage tank 93 and passing it through line 94, valve 85 and line 9| into the silicia gel immediately following each addition of sidestream fraction from line 88.

After introduction of the desired amount of butane, xylene which has been obtained from tank 19 by mean's of line 80 is passed through line 98, valve 91 and line 8| into the adsorbent in amount effective to displace the benzene and toluene. Following the introduction of the xylene desorbent, another portion of sidestream fraction from line 88 is introduced into adsorption zone 92. During this time lthe xylene remaining 'in the silica gel is displaced from it by the benzene and toluene contained in the sidestream fraction.

The elux stream, passing from adsorption zone 92 through line 98 is, during each cycle, separated into two portions. During a part of the cycle the eillux is composed mainly of butane, benzene, toluene and xylene, and this portion is sent through valve 99 and line |00 to distillation column Butane is recovered through overhead line |02 and is returned to storage tank 83. Purified benzene is obtained through line |03 and passes to storage tank |04 from which it may be withdrawn through line |05 as a product of the process. Toluene and xylene are removed together from column |0| by means of line |06 vand the mixture is sent through line |01 into another distillation column |08. The bottom product from distillation column 85, 'which is composed mainly of xylene but contains some toluene, is introduced from line 89 into line |01 and also sent into column |08. The mixture is therein distilled to obtain purified toluene through overhead line |09, the toluene then passing to storage tank ||0 from which it may be withdrawn via line as another product of the process. From the base of column |08 xylene is recovered and is sent through lines ||2 and I3 back to storage tank 18.

The other portion of eflluent from adsorption zone 92 during each cycle is'composed essentially of butane and xylene together with the saturate hydrocarbons that were present in the sidestream fraction in line 88. This portion flows through valve ||4 and line ||5 into distillation column I6. Butane is distilled overhead and is returned by means of line ||1 to tank 93. Xylene is recovered as a bottom product and passes through lines ||9 and ||3 back to storage tank 18. The

saturate hydrocarbons present in this efflux portion are removed from column ||6 as a sidestream via line ||8. In conducting this distillation it is an important feature for commercial practice to adjust the fractionation in column I6 so that a minor proportion of the butane will pass out of the column along with the sidestream fraction in line ||8 rather than all overhead. This( will insure the removal of hydrocarbons boiling between the boiling points of butane and benzene and will prevent a build-up in concentration of such hydrocarbons within the system during prolonged operation. The incluson of a minor amount of butane in the saturate material produced through line ||8 will not be objectionable for refinery practice, since such material generally will be used for making motor fuel in which the presence 'of a minor amount of butane is desirable.

The heavier naphtha charge, which illustratively has a boiling range of 24U-275 F., is separately reacted in another catalytic reforming step generally similar to that previously described for the lighter naphtha charge. Naphtha charge, containing Cs naphthenes, enters the system through line |30 and is mixed with a recycled hydrogen stream from line |3 the mixture then passing into catalytic reformer |32 for conversion of thenaphthenes to aromatic hydrocarbons. Reaction conditions for this reforming step generally should include a temperature of 800-1000 F., a pressure of 3D0-650 lbs/square inch, a space rate of 1-4.` liquid volumes oi catalyst per hour and a hydrogen recycle ratio of 2-7 moles'per mole ofl naphtha charged. More preferable conditions include a temperature of 900-950" F., a pressure of 450-550 lbs/square inch, a space rate of 1.5-3 liquid volumes of naphtha per volume of catalyst per hour and a hydrogen recycle ratio of 3-5 moles per mole of naphtha charged.

'Ihe reaction mixture from reformer |32 passes through line |83 and cooler |34 and into separator |35 wherein liquid product is separated from gaseous product. The gaseous stream flows from the top of the separator through line |36 and a portion may be recycled through valve |31 and lines |38 and |3|. The remainder is introduced into absorption zone |39 wherein it flows upwardly countercurrent to xylene obtained from tank 19 and supplied to the absorber lby means of lines |40 and |4|. Excess hydrogen and unabsorbed hydrocarbons are removed from the top of absorber |38 through line |42.

The liquid product from the bottom of separator |35 ows through line |43 and is mixed with the xylene stream from the bottom of absorber |39. The mixture is then introduced through line |44 into distillation column |45. Hydrocarbons boiling below the xylene boiling while hydrocarbons ltal number :,ssarvs 9 are distilled overhead through line |49.

boiling above the xylene boiling range are obtained as bottom product which is passed through line |41 to tank |49. The latter product is composed mainly oi Cn aromatica and generally is obtained in relatively small proportion compared to the amount of xylene produced.

As a sidestream from column |45 a xylenerich product is obtained through line |41 and is sent to storage tank 19. Inasmuch as the xylene in tank 19 is utilized in other parts of the process as previously described, there is a tendency for saturate hydrocarbons which boil above the boiling point or toluene to be present in the xylene returned to tank 19 through line III. It is particularly important in commercial practice to prevent accumulation oi such material in the system; otherwise the material will contaminate the xylene withdrawn as a product and also, in order to escape from the system upon continued operation of the process. will vcontaminate the toluene product. Accordingly, an important feature of the process of Figure 2 is the circulation of xylene from tank 19 through line |40 for use in absorber |39 and then back to distillation column |45. v. This insures removal of thosev hydrocarbons which boil between toluene and xylene through overhead line |49 and prevents their accumulation in the system. A by-pass line around absorber |39, which is shown as line |50 having valve |51, preferably is provided so that xylene from tank 19 may be recycled to distillation column |45 in an amount in excess of that required in absorber |99. By operating in the described manner the sidestream xylene product in line |49 will be substantially purer than the xylene in tank 19. Accordingly xylene is withdrawn as another product of the process preferably from line |49 by means of line |52.

It will be understood that the terms Ca naphthenes, C1 naphthenes" and "Ca naphthenes as used herein refer to naphthenes having a tooi carbon atoms per molecule of respectively, regardless of range six, seven and eight,

` rich product .to said source of supply,

aromatlc hydrocarbons and xylene and another of whichis composed mainly of benzene, toluene and xylene, distilling each xylene-rich product therefrom,

portion to wrecover returning vxyleneand removing from the system xylene-rich product containing xylene used in said cyclic operation to thereby prevent progressive contamination or xylene by saturate hydrocarbons boiling above toluene.

2. Method according to claim 1 wherein the adsorbent during said cyclic operation is conwhether the number of carbon atoms in the naphthene ring is five or six.

I claim: I

1. Method of producing high purity aromatic hydrocarbons which comprises distilling a petroleum'stock to produce a fraction containing Ce and C1 naphthenes and another fraction containing Ca naphthenes and boiling essentially below the boiling range oi xylenes, contacting the inst-mentioned fraction with a dehydrogenating catalyst under conditions eiIective to convert contained naphthenes to benzene and tolueneJ contacting the second-mentioned fraction with a dehydrogenating catalyst under conditions effective to convert contained naphthenes to xylenes, distilllng reaction product resulting from the second contacting step to separate a xylene-rich product, passing the xylene-rich productv to a source of supply, and recovering toluene from reaction product oi the first contacting step by means of a cyclic operation in -each cycle of which such reaction product is contacted with an aromatic-selective adsorbent .benzene and tacted with butane, immediately following contact with the reaction product of the iirst contacting step, in amount to remove associated non-aromatic hydrocarbons trom the adsorbed benzene and toluene without substantial displacement oi benzene and toluene from the adsorbent.

3. Method of producing high purity aromatic hydrocarbons which comprises distillxng a petroleum stock to produce a fraction boiling essentially belowl 230 F. and containing Ce and C1 naphthenes and a heavier fraction boiling essentially below 275 F., contacting the first-mentioned fraction with a dehydrogenating' catalyst under conditions effective to convert contained naphthenes to benzene and toluene, distilling the reaction product to separate hydrocarbons boiling essentially below 230 F. from a toluene-rich fraction boiling 'essentially above 230 F.. contacting said heavier fraction with a dehydrogenating catalyst under conditions eective to convert contained naphthenes to aromatics, distilling the resulting reaction mixture to separate a xylenerich product. passing the xylene-rich product to a source of supply, and recovering benzene and toluene from said separated .hydrocarbons boiling essentially below 230 F. by means of cyclic operation in each cycle of which the said separated hydrocarbons are contacted with an aromatic-selective adsorbent to adsorb benzene and toluene while displacing xylene remaining in the adsorbent from the preceding cycle, after which the adsorbent is contacted withV xylene-rich product from said source of supply to .displace thel benzene and toluene, collecting eiliuent from the adsorbent in two portions one of which is composed mainly of non-aromatic hydrocarbons and xylene and another of which is composed mainly of benzene, toluene and xylene, distilling each portion to recover xylene-rich product therefrom, returning xylene-rich product to said source oi supply, and removing from the system xylenerich product containing xylene used in said cyclic operation to thereby prevent progressive contamination of xylene by saturate hydrocarbons boiling above toluene. l

4. Method accordingv to claim 3 wherein the adsorbent during said cyclic operation is contacted with butane, immediately following contact with the said separated hydrocarbons, in amount to remove associated -non-aromatic hydrocarbons from the adsorbed benzene and toluene without substantial displacement of benzene and toluene lfrom the adsorbent.

5. Method of producing high purity aromatic hydrocarbons which comprises fractionating a hydrocarbon stock to produce a fraction boiling essentially within the range 15o-225 F. and a heavier fraction boiling above toluene and essentially below 275 F., contacting the firstmentioned fraction with a dehydrogenating catalyst under conditions effective to convert contained naphthenes to benzene and toluene, distillrich fraction boiling essentially above 230 F., contacting said heavier fraction with a dehydrogenating catalyst under conditions eil'ective to convert contained naphthenes to aromatics, distilling the resulting reaction mixture to separate hydrocarbons boiling essentially below 275 F. and to yield a xylene-rich product and a heavier product rich in C9 aromatics, passing the xylene-rich product to a source or supply, and recovering benzene and toluene from said. separated hydrocarbons boiling essentially below 230 F. by means of cyclic operation in each cycle of which the said separated hydrocarbons are contacted with an aromatic-selective adsorbent to adsorb benzene and toluene while displacing xylene remaining in the adsorbent from the preceding cycle, after which the adsorbent is contacted with xylene-rich product from said source of supply to displace the benzene and toluene, collecting eiliuent from the adsorbent in two portions one oi' which is composed mainly of non-aromatic hydrocarbons and xylene and another of which is composed mainly of benzene, toluene and xylene. distilling each portion to recover xylenerich product containing xylene used in said' cyclic operation to thereby prevent progressive contamination of xylene by saturate hydrocarbons boiling above the toluene.

ing the xylene-rich product to a source of supply,

rich product therefrom, returning xylene-rich product to said source of supply, and removing from the system xylene-rich product containing xylene used in said cyclic operation to thereby prevent progressive contamination of xylene by saturate hydrocarbons boiling above toluene.

6. Method accordingl to claim 5 wherein the adsorbent during said cyclic operation is contacted with butane, immediately following contact with the said separated hydrocarbons, in amount to remove associated non-aromatic hydrocarbons from the adsorbed benzene and toluene without substantial displacement of benzene and toluene from the adsorbent.

7. In a process, for producing high purity aromatics the steps which comprise contacting a petroleum fraction of relatively low boiling range containing naphthenes within the Ce and Cv group with a dehydrogenating catalyst under conditions effective to convert contained naphthenes to aromatic hydrocarbon of 6-7 carbon atoms, contacting a second petroleum fraction of relatively high boiling range such as to include Ca naphthenes but below the boiling range of xylenes with a dehydrogenating catalyst under conditions effective to convert Cs naphthenes to xylenes, distilling reaction product resulting from the second contacting step to separate a xylene-rich product. passing the xylenerich product to a source of supply, and recovering aromatic hydrocarbon of 8 7 carbon atoms from reaction product of the first contacting step by means of a cyclic operation in each cycle of which such reaction product is contacted with an aromatic-selective adsorbent to adsorb aromatic hydrocarbon o! 6-7 carbon atoms while displacing. xylene remaining in the adsorbent from the preceding cycle, after which the adsorbent is contacted with xylene-rich product from said source of supply to displace the aromatic hydrocarbon o! 6-7 carbon atoms, collecting eilluent from. the adsorbent in two portions one oi' which is composed mainly of non-aromatic hydrocarbons and xylene and another of which is composed mainly oi' aromatic hydrocarbon of 6-7 carbon atoms and xylene, distilllng each portion to recover xylene-rich product therefrom, returning xylene-rich product to said source of supply, and removing from the system xyleneand recovering benzene from reaction product of the iirst contacting step by means of a cyclic operation in each cycle ofwhich such reaction product is contacted with an aromatic-selective adsorbent to adsorb benzene from associated nonaromatic hydrocarbons while displacing xylene remaining in the adsorbent from' the preceding cycle, after which the adsorbent is contacted with xylene-rich product from said source of supply to displace the benzene, collecting eilluent from the adsorbent in two portions one of which is composed mainly of non-aromatic hydrocarbons and xylene and another of which is composed mainly of benzene and xylene, distilling each portion to recover xylene-rich `product therefrom, returning xylene-rich product to said source oi' supply, and removing from the system xylene-rich product containing xylene used in said cyclic operation to thereby prevent progressive contamination of xylene by higher boiling saturate hydrocarbons from reaction product of said contacting steps.

9. In a process for producing high purity aromatics the steps which comprise contacting a petroleum fraction of relatively low boiling range such as to include C7 naphthenes with a dehydrogenating catalyst under conditions eiective to convert C7 naphthenes to toluene, contacting a second petroleum fraction of relatively high boiling range such as to includelCa naphthenes but below the boiling range of xylenes with a dehydrogenating catalyst under conditions eiective to convert Ca naphthenes to xylenes, distilling reaction product resulting from the second contacting step to separate a xylene-rich product. passing the xylene-rich product to a source of supply, and recovering toluene from reaction product of the rst contacting step by means of a cyclic operation in each cycle of which such reaction product is contacted with an aromatic-selective adsorbent to vadsorb toluene from associated nonaromatic hydrocarbons while displacing xylene remaining in the adsorbent from the preceding cycle, after which the adsorbent is contacted with xylene-rich product from said source of supply to displace the toluene, collecting eiliuent yfrom the adsorbent in two portions one of which is composed mainly of non-aromatic hydrocarbons and xylene and another of which is composed mainly of toluene and xylene, distilling each portion to recover xylene-rich product therefrom, returning xylene-rich product to said source of supply, and removing from the system xylene-rich product containing xylene used in said cyclic operation to thereby prevent progressive contamination oi' xylene by saturate hydrocarbons boiling above the boiling point oi.' toluene.

l0. In a process for producing high purity aromatics the steps which comprise contacting a petroleum fraction of relatively low boiling range such as to include C; 'and Cv naphthenes with a dehydrogenating catalyst under conditions effective to convert the naphthenes .to benzene andv toluene, contacting a second petroleum'fraction of relatively high boiling range such as to include 'Ca naphthenes but below the boiling range of ceding cycle, after which the adsorbent is con-- tacted with xylene-rich product from said source of supply to displace the benzneand toluene, collecting eiiluent from the adsorbent in two portions one of which is composed mainly' of nonaromatic hydrocarbons vand xylene and another s of which is composed mainly of benzene, toluene and xylene,` distilling each portion to recover xylene-rich product therefrom, returning xylenerich product to said source of supply, and removing from the system xylene-rich product containf ing xylene used in said cyclic operation to thereby prevent progressive contamination of xylene by saturate hydrocarbons boiling above the boiling point of toluene.

11. Process according to claim 10 wherein the relatively low boilingpetroleum fraction boilsessentially within the range of 15o-225 F. and the relatively high boiling petroleum fraction boils essentially within the range of 240-2'75`F.

12. Method of roducing high purity aromatics which-comprises: contacting a petroleum fraction of relatively low boiling range such as to include C6 and Cv naphthenes but essentially below thev boiling point of toluene with a dehydrogenating catalyst in the presence of free hydrogen under conditions effective to convert the naphthenes tov benzene and toluene; cooling the reaction mixture and separating a liq'uid phase from a gaseous phase; passing gaseous phase countercurrent to hereinafter specified xylene-rich product to absorb desired heavier components therefrom; distilling a mixture of said liquid phase and the xylene-rich product carrying absorbed components to obtain a light fractio vboiling essentially below benzene, an intermediate fraction containing benzene and part of the toluene produced in the reaction, and a heavier fraction composed mainly of toluene and xylene; separating benzene and toluene from said intermediate fraction by means of a cyclic operation in each cycle of which the intermediate fraction is introduced intoa bedof aromaticselective adsorbent to adsorb benzene and toluene, butane is then introduced into the adsorbent in amount to remove associated non-aromatic hydrocarbons from the adsorbed benzene and toluene without substantial displacement of benzene and toluene from the adsorbent, and another portion of the said specified xylene-rich product is then introduced into the adsorbent to displace the benzene and toluene; collecting the elliuent from the adsorbent bed during each cycle in two portions one of which is composed' mainly i4 of butane, xylene and non-aromatic hydrocarbons derived from said intermediate fraction and the other of which is composed mainly of benzene, toluene and xylene; distilling said one portion to vrecover a butane fraction. an intermediate fraction rich in said non-aromatic hydrocarbons, and a xylene-rich fraction; distilling said other portion to recover a benzene-rich product and a fraction'composed mainly of toluene and xylene; distilling the last-mentioned fraction in admixture with the said heavier fraction composed mainly of toluene and xylene -to yield a toluene-rich product and to recover xylene-rich product; contacting a second petroleum fraction of relatively high boiling range suchv as to include C'naphthenes but essentially below the boiling rangev of xylene's with a dehydrogenating catalyst in the presence of free hydrogen under conditions eilective to convert.

Ca naphthenes to xylenes; cooling the resulting reaction mixture and separating a liquid phase from agaseous phase; passing gaseous phase counter-current to hereinafter specified xylenerich product to absorb desired heavier components therefrom; and distilling a mixture of said liquid plase and the resulting xylene-rich product carrying absorbed components to obtain a relatively light 'fraction boiling essentially below the xylene boiling range and composed mainly of non-aromatic hydrocarbons, an intermediate fraction constituting thesaid speciiied xylenerich product, and a heavier vfraction containing -aromatics boiling above the xylene boiling range.

13. Method-according to claim 12 wherein the distillation of the said one portion of eiliuent from the adsorbent bed is conducted in a manner so as to include a minor portion of the containedl butane in the said intermediate fraction produced therefrom to thereby prevent progressive contamination of the butane by sat- `urate hydrocarbons vboiling between butane and benzene.

14. Method of producing high purityraromatics which comprises: contacting a petroleum fraction of relatively low boiling range such as to include Ce and C1 naphthenes but essentially below the boiling point of toluene with a dehydrogenating catalyst in\the presence of free hydrogen under conditions effective to convert the naphthenes to benzene and toluene; cooling the reaction mixture and separating a liquid phase from a gaseous phase; passing gaseous phase countercurrent to a xylene-rich product obtained from the hereinafter specied source of supplyl to absorb desired heavier components therefrom; distilling a mixture of said liquid phase and the xylene-rich product carrying absorbed components to obtain a light fraction boiling essentially below benzene. an intermediate fraction containing benzene and part of the toluene produced in the reaction, and a heavier fraction composed mainly of toluene and xylene; separating benzene and toluene from said intermediate fraction by means of a cyclic operation in each cycle of which the intermediate fraction is introduced into a bed of aromaticselective adsorbent to adsorb benzene and toluene, butane cycled from a butane supply source is then introduced into the adsorbent in amount to remove associated non-aromatic hydrocarbons from the adsorbed benzene and toluene without substantial displacement of benzene and' awaits 15 into the adsorbent to displace the benzene and toluene; collecting the effluent from the adsorbent bed during each cycle in two portions one of which is composed mainly of butane, xylene and non-aromatic hydrocarbons derived. from said intermediate fraction and the other of which is composed mainly of benzene, toluene and xylene; distilling said one portion to recover a butane fraction, an intermediate fraction rich in said non-aromatic hydrocarbons and containing a minor portion of butane so as to prevent progressive contamination of the recovered butane by saturate hydrocarbons boiling between butane and benzene, and a xylene-rich fraction; returning recovered butane to said butane supply source; distilling said other portion to recover a benzene-rich product and a frac-` -boiling range of xylenes with a dehydrogenating catalyst in the presence of free hydrogen under conditions effective to convert Ca naphthenes to xylenes; cooling the resulting reaction mixture and separating a liquid phase from a gase ous phase; passing gaseous phase countercurrent to xylene-rich product obtained from said specified source of supply to absorb desired heavier components therefrom; distilling a mixture of said liquid phase and the resulting xylenerich product carrying absorbed components to obtain a relatively light fraction boiling essentially below the xylene boiling range and composed mainly of non-aromatic hydrocarbons, an intermediate fraction constituting the xylenerich product, and a heavier fraction containing aromatics boiling above the xylene boiling range; and feeding a portion of such intermediate fraction to said specified source of supply while withdrawing the remainder as xylene-rich product of the process, whereby progressive contamination, by saturate hydrocarbons boiling above toluene, of the xylene circulated to the adsorbent bed is prevented and the withdrawn xylene-rich product is of a purity higher than the average purity of' the xylene-rich product in said source of supply.

WILLIAM H. DAVIS.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,375,573 Meier May 8, 1945 2,398,101 Lipkin Apr. 9, 1946 2,464,311 Hiatt et al. Mar. 15, 1949 i 2,496,253 Purcell et al Jan.l31, 1950 2,564,717 Olsen Aug. 21, 1951 

1. METHOD OF PRODUCING HIGH PURITY AROMATIC HYDROCARBONS WHICH COMPRISES DISTILLING A PETROLEUM STOCK TO PRODUCE A FRACTION CONTAINING C6 AND C7 NAPHTHENES AND ANOTHER FRACTION CONTAINING C8 NAPHTHENES AND BOILING ESSENTIALLY BELOW THE BOILING RANGE OF XYLENES, CONTACTING THE FIRST-MENTIONED FRACTION WITH A DEHYDROGENATING CATALYST UNDER CONDITIONS EFFECTIVE TO CONVERT CONTAINED NAPHTHENES TO BENZENE AND TOLUENE, CONTACTING THE SECOND-MENTIONED FRACTION WITH A DEHYDROGENATING CATALYST UNDER CONDITIONS EFFECTIVE TO CONVERT CONTAINED NAPHTHENES TO XYLENES DISTILLING REACTION PRODUCT RESULTING FROM THE SECOND CONTRACTING STEP TO SEPARATE A XYLENE-RICH PRODUCT PASSING THE XYLENE-RICH PRODUCT TO A SOURCE OF SUPPLY, AND RECOVERING BENZENE AND TOLUENE FROM REACTION PRODUCT OF THE FIRST CONTACTING STEP BY MEANS OF A CYCLIC OPERATION IN EACH CYCLE OF WHICH SUCH REACTION PRODUCT IS CONTRACTED WITH AN AROMATIC-SELECTIVE ADSORBENTN IS CONTO ADSORB BENZENE AND TOLUENE WHILE DISPLACING XYLENE REMAIMING IN THE ADSORBENT FROM THE PRECEDING CYCLE, AFTER WHICH THE ABSORBENT IS CONTACTED WITH XYLENE-RICH PRODUCT FROM THE SOURCE TO SUPPLY A DISPLACE THE BENZENE AND TOLUENE COLLECTING EFFIUENT FROM THE ADSORBENT IN TWO PORTIONS ONE OF WHICH IS COMPOSED MAINLY OF NONAROMATIC HYDROCARBONS AND XYLENE AND ANOTHER OF WHICH IS COMPOSED MAINLY OF BENZENE, TOLUENE AND XYLENE, DISTILLING EACH PORTION TO RECOVER XYLENE-RICH PRODUCT THEREFROM, RETURNING XYLENERICH PRODUCT TO SAID SOURCE OF SUPPLY, AND REMOVING FROM THE SYSTEM XYLENE-RICH PRODUCT CONTAINING XYLENE USED IN SAID CYCLIC OPERATION TO THEREBY PREVENT PROGRESSIVE CONTAMINATION OF XYLENE BY SATURATE HYDROCARBONS BOILING ABOVE TOLUENE. 